(1) Field of the Invention
The present invention relates to a dry etching apparatus and a dry etching method.
(2) Description of Related Art
FIG. 10 schematically shows the structure of a known dry etching apparatus for use in etching of oxide films or other films.
As shown in FIG. 10, a lower electrode 102 is provided on the bottom of a reaction chamber 101, and an upper electrode 110 is provided at the ceiling of the reaction chamber 101 to face the lower electrode 102. The lower electrode 102 and upper electrode 110 are separated from each other by as large a space as necessary to generate plasma. A focus ring 108 is placed on the lower electrode 102 to hold a to-be-processed substrate 103. The focus ring 108 has, at its middle part, a cavity for holding the to-be-processed substrate 103.
An RF power supply 104 is provided outside of the reaction chamber 101 to apply voltage to the lower electrode 102. The RF power supply 104 applies power at 13.56 MHz to the lower electrode 102. On the other hand, the upper electrode 110 is connected to a ground potential 107. A gas inlet 105 is provided at the top of the upper electrode 110 to introduce a process gas into the reaction chamber 101. The process gas is supplied through the gas inlet 105 and an unshown conduit passing through the upper electrode 110 into the reaction chamber 101. On the other hand, a gas outlet 106 is provided at the lower part of the reaction chamber 101 to let out the process gas from the reaction chamber 101.
Next, a dry etching method using the known dry etching apparatus as shown in FIG. 10 will be described with reference to FIG. 11. A description will now be given of, as an example, a process step of forming a contact hole in an interlayer insulating film by using a silicon nitride (SiN) film as an etching stopper. FIG. 11 is a cross sectional view showing the state of an SRAM memory cell where a contact hole for electrically connecting an upper interconnect to a substrate is formed partway between two of adjacent gate electrodes.
As shown in FIG. 11, in a semiconductor device before the process step of forming a contact hole, a gate oxide film 115 formed by thermal oxidization and a gate electrode 118 located thereon are placed on the top surface of a silicon substrate 117 in which a well or isolation is previously formed. While an offset oxide film 113 is formed on the top surface of the gate electrode 118, a sidewall consisting of a film 116 of L-shaped cross section and a coating film 120 located on the L-shaped film 116 is formed on the side surface of the gate electrode 118. Furthermore, unshown source/drain regions each having an LDD structure are provided in the surface of the silicon substrate 117. More particularly, there are provided unshown extension diffusion layers self-aligned with the gate electrode 118 and source/drain regions self-aligned with the sidewall.
In this state, a SiN etching stopper film 114 is formed on the silicon substrate 117 to cover the offset oxide film 113 and the surface of the sidewall. An interlayer insulating film 112 made of SiOx (x=1, 2 . . . ) is deposited on the SiN etching stopper film 114. Thereafter, the surface of the interlayer insulating film 112 is planarized. Subsequently, a resist pattern 119 is formed on the interlayer insulating film 112 by photolithography. Next, the interlayer insulating film 112 is subjected to dry etching using the resist pattern 119 as a mask. When the selectivity of SiOx constituting the interlayer insulating film 112 to the SiN etching stopper film 114 is large enough during this dry etching, the etching rate rapidly decreases at the moment that a contact hole passes through the interlayer insulating film 112 and reaches the SiN etching stopper film 114. At this moment, if the process is switched to etching conditions on which SiN can selectively be removed, a contact hole can be completed.
However, SiN and SiOx have the following etching characteristics. The energy of chemical bonds of atoms constituting a crystal lattice of SiN is similar to that of SiOx, and SiN and SiOx have basically the same etching species. In addition, when fluorine radicals are used for etching, the etching rate of SiN is a little faster than that of SiOx. In view of these points, the high-selectivity etching of a SiOx film using SiN as an etching stopper film is considered to have an extremely high difficulty level.
The currently dominating method for dry etching of interlayer insulating films is a method in which a CF-based polymer is formed using plasma of a fluorocarbon-based gas to ensure the selectivity. The use of a fluorocarbon-based gas permits the generation of a CF-based polymer, and the CF-based polymer is deposited on the surface of the underlying interlayer insulating film to form a CF-based protective film. In order to compensate for decrease in the etching rate of the interlayer insulating film due to the deposition of the CF-based protective film, a large amount of etching species are generated in high-density plasma. In this way, in order to obtain a high selectivity, it is very important to generate high-density plasma.
The dry etching apparatus shown in FIG. 10 has been demanded to have improved uniformity of the etching rate. To meet this demand, a method has been suggested in which an upper electrode is curved to have a concave shape (see Japanese Unexamined Patent Publication No. 11-317396). The reason for this is as follows. Reaction products are likely to be deposited on the peripheral part of a to-be-processed substrate in a reaction chamber due to the convection of an etching gas. Since the etching rate is therefore decreased in the peripheral part of the to-be-processed substrate, this deteriorates the uniformity of the etching rate of the whole to-be-processed substrate. To cope with this, as disclosed in Japanese Unexamined Patent Publication No. 11-317396, the upper electrode is curved to have a concave shape, and thus the electrode-to-electrode distance is shortened in the peripheral part of the to-be-processed substrate. In this way, the etching rate of the peripheral part can be enhanced. Thus, the etching rate of the peripheral part of the to-be-processed substrate can be made substantially equal to that of the middle part of the to-be-processed substrate. In other words, the uniformity of the etching rate can be enhanced by curving the upper electrode to have a concave shape.
By the way, in recent years, in order to increase the number of obtainable devices per substrate and reduce cost, the diameter of a to-be-processed substrate has been increased. With the increase in the diameter of the to-be-processed substrate, the diameters of electrodes in an etching apparatus become larger.
Meanwhile, high integration of semiconductor devices has provided finer design rules, and in order to more precisely perform etching, the controllability of plasma to be generated during etching must be enhanced. To meet this demand, the electrode-to-electrode distance in an etching apparatus need be shortened.
As can be seen from the above, while the diameters of electrodes tend to increase, the electrode-to-electrode distance is becoming shorter. However, the use of such an etching apparatus causes an inconvenience as described hereinafter. FIGS. 12A through 12C are cross-sectional views for explaining a problem on the formation of a contact hole in a large-diameter to-be-processed substrate under the use of an etching apparatus having a small electrode-to-electrode distance.
As shown in FIG. 12A, a resist pattern 122 having an opening 123 of, for example, a contact hole pattern is formed on an oxide film 121 serving as an object to be etched. In this case, the diameter of the opening (dimension after lithography) in the resist pattern 122 is represented as L0.
The oxide film 121 as shown in FIG. 12A is etched using the resist pattern 122 as a mask, thereby forming a contact hole in the oxide film 121. FIG. 12B illustrates the shape of a contact hole 123a on the middle part of the to-be-processed substrate, and FIG. 12C illustrates the shape of a contact hole 123b on a peripheral part of the to-be-processed substrate. In FIGS. 12B and 12C, the respective diameters of the tops of the opened contact holes (dimensions after dry etching) are represented as L1 and L1′ and the respective diameters of the bottoms thereof as L2 and L2′.
With the use of the known dry etching apparatus, the contact hole 123a on the middle part of the to-be-processed substrate is tapered as shown in FIGS. 12B and 12C. As a result, the inside diameter of the contact hole becomes smaller with the increase in depth. This deteriorates the uniformity of the hole diameters in the surface region of the to-be-processed substrate.